Crossbreeding experiments are methods for experimentally verifying and applying Mendel's laws. In particular, the test cross is a powerful method for determining an unknown genotype and is widely used in applications ranging from agricultural breeding to gene mapping. Let us learn about various inheritance patterns and methods for distinguishing them.
Basic Knowledge
Test cross: An individual showing the dominant phenotype is crossed with a homozygous recessive (aa) individual to determine whether its genotype is homozygous (AA) or heterozygous (Aa). ① If all offspring are dominant → the parent is AA. ② If the offspring show dominant:recessive = 1:1 → the parent is Aa. This is essential for confirming pure lines in agricultural breeding.
Incomplete dominance (codominance): A case in which F₁ shows an intermediate trait different from both the dominant and recessive traits. Example: red flowers (RR) × white flowers (rr) → pink flowers (Rr). In F₂, red:pink:white = 1:2:1. Codominance: A heterozygote simultaneously shows both traits. Example: IᴬIᴮ in the ABO blood group = type AB.
Lethal gene: A gene that causes death when homozygous. Example: the agouti gene in yellow mice (Ayay = yellow・Ay Ay = embryonic lethal) → yellow:wild type = 2:1 (the 3:1 ratio expected if lethal individuals did not disappear).
Interactions among multiple genes: Complementary gene action (a trait is expressed only when two genes are both present), epistasis (one gene suppresses the expression of another), and cumulative gene action (multiple genes contribute quantitatively to the same trait). These produce segregation ratios other than 9:3:3:1.
A test cross of individual X with a dominant phenotype produces offspring in a dominant:recessive ratio of 1:1. State the genotype of X and explain this result in connection with the law of segregation.
Answer: The genotype of X is Aa (heterozygous). The test-cross partner is aa. In the cross between Aa and aa, A and a gametes each account for 1/2 of the gametes and are fertilized → Aa (dominant phenotype):aa (recessive phenotype) = 1:1. This directly shows that A and a segregate at equal frequencies during gamete formation in Aa, as stated by the law of segregation.
- Test cross = cross with a homozygous recessive (aa) to determine the genotype
- Test-cross results: all dominant → AA / 1:1 → Aa
- Incomplete dominance = F₁ has an intermediate trait; F₂ is 1:2:1
- Codominance = both traits are expressed simultaneously in a heterozygote (type AB blood)
- Lethal gene = homozygous lethality shifts the segregation ratio
- Epistasis = one gene masks another
- Complementary gene action = a trait appears only when two genes are both present
Points to Note
① The partner in a test cross must always be homozygous recessive (aa). ② In incomplete dominance, the distinction between dominant and recessive becomes unclear, so the correspondence between genotype and phenotype must be written accurately. ③ Depending on the direction of epistasis (which gene masks the other), the F₂ ratio changes in various ways, such as 12:3:1, 9:3:4, and 15:1.
Practice
- Explain the procedure of a test cross and how to determine the genotype from its results.
- State the F₂ segregation ratio in incomplete dominance in comparison with complete dominance.
- Explain why the usual 3:1 segregation ratio is altered when a lethal gene is present.